1. Field of the Invention
The present invention relates to a method for detecting and locating mobile craft (aircraft for example as well as surface vessels, underwater vessels, vehicles, etc.), in using especially the radiation produced by transmitters present in the vicinity for other applications (here below called transmitters of opportunity).
It can be applied to any communications transmitter or any transmission system using waveforms with a known synchronization protocol on one or more carrier frequencies exploited by the transmitter.
It can be used especially with systems comprising digital transmitters dedicated to video broadcasting (such as DVB-T or Digital Video Broadcasting-Terrestrial) or audio broadcasting (DAB or Digital Audio Broadcasting) or again cell network infrastructure transmitters (for PMR or Private Mobile Radio).
2. Description of the Prior Art
The majority of the presently existing or planned solutions for locating aircraft are based on general principles of radars, radar interceptors or IFF (Identification Friend or Foe) systems in electromagnetics and principles of sonars and sonar interceptors in acoustics.
Generation Controlled by a Transmitter Planned for this Purpose
The radar or sonar type solution uses a wave which is intended to be reflected from obstacles (aircraft, etc.). This solution exploits the paths taken by signals reflected by a receiver working together with the transmitter:    either collocated with the transmitter, in the case of active sonar or monostatic sonar,    or delocated from the transmitter in the form of:    active sonar or bistatic radar using one transmitter and one receiver    multistatic radar comprising several transmitters and one or more receivers.
These solutions nevertheless have the drawback of using specific transmitters that collaborate closely with the reception system of the signal induced by the presence of the aircraft. Such transmitters are indiscreet and often costly, and require much space. Furthermore, they are generally dedicated to certain special applications. They are small in number. They often require large amounts of space and their availability is sometimes limited.
Exploitation of the Proper Radiation Produced
Other solutions use the proper radiation produced:    by transmitter specific to the mobile craft. This is the well-known principle used in IFF and air traffic control systems to identify and track aircraft. This principle is also used in radar interceptors or sonar interceptors.    by the mobile craft itself (using for example passive sonar, detection of magnetic or electromagnetic anomalies). This radiation may come directly from the mobile craft, or may result from a phenomenon of the masking of a radiation source (shadow), or more generally it may come from a temporary and local modification of the ambient electromagnetic or acoustic/seismic field: the principle of anomaly detection applied for example for the activation of mine seekers for certain surveillance sensors, etc.
These approaches have the following drawbacks:    they are passive, without any collaboration with the transmitter;    they work only in the case of transmissions effectively detectable by a reception system, thus raising “rendezvous” problems, causing random phenomena, requiring a learning process, etc.
Furthermore, these approaches produce a rough measurement whose nature is chiefly angular. The distance is obtained only after passing through a tracking filter under certain conditions of implementation that are generally restricted.
Exploitation of the Proper Radiation Sent by Opportunity Transmitters and Reflected by the Targets
Such methods are implemented to exploit the fortuitous presence of radar or other transmitters, for purposes of detection of targets present in the environment by receivers planned for this purpose. They rely on classic measurements of time difference of arrival, (TDOA) or direction of arrival (DOA) and sometimes on
Doppler Subtraction Techniques.
These approaches presently have the following drawbacks:                the planned or existing systems are based essentially on the waveform properties and the Doppler properties of the waves sent (often multicarrier waves).        The planned or existing systems generally necessitate the simultaneous reception of signals of the direct path or the path of the signals reflected from the target to determine a difference between the instants of arrival.        Today, none of the systems possesses efficient capacities for separating the transmitters of opportunity that are present. For this reason:                    the transmitters of opportunity envisaged in many existing systems are very small in number (in general, there is only one per processing band). This has the effect of limiting the locating precision produced or of considerably extending the time period needed for the production of high-quality locating.            The existing reception systems capable today of processing transmitters of opportunity are hard to integrate into relatively dense transmitter networks sharing the same frequency resource for such as networks designed for radio broadcasting (DVB-T, DAB) or cell network transmission (GSM or Global System Mobile, UMTS or Universal Mobile Telecommunication System), while the density of transmitters is a notable factor in performance (entailing numerous measurements and geometrical configurations).                        
The current systems do not enable the efficient processing of situations of complex multiple-transmission and are even less suited to the processing of situations of interference between the received signals.
The idea of the invention lies notably in the exploitation of the presence of transmitters of certain radio and television broadcasting networks such as the DVB-T, DAB broadcasting systems or the infrastructure transmitters of cell networks, PMR or the like. It uses especially opportunities for the time-related synchronization of signals, as well as efficient source-separation methods.
The invention uses especially the protocol and the synchronization sequences known in certain digital audio or digital video broadcasting systems or again in cell networks to separate the transmitters and the signals directly determine the instants and directions of arrival of the signals of the different propagation paths and the pulse response of the propagation filter.